This invention relates to apparatus which may be operated when in contact with a portion of the operator's body such as his or her hands and includes protective circuitry therein.
More particularly, this invention relates to protective circuitry incorporated in an appliance such as a portable hand-held hair dyer, tool, or the like which protects the operator or user from electrocution.
It is known in the art that various devices and circuits are available which will prevent the operator of an appliance typically of the hand-held variety from an electrical shock which ultimately may result in the operator's death. Most typically, these electrocution proof devices utilize a differential current transformer which senses a small difference in current in normally balanced power lines or cables. Such a difference is typically caused by a leakage of current from one of the line conductors to ground, thus depriving the return line of some of its normal current which would establish a balance or zero difference in current at an associated sensor. When the differential current is below a predetermined level, typically about 0.005 amperes, the power is normally allowed to flow uninterrupted. If a larger differential current occurs, the circuit is interrupted since a potential death hazard to a human being is occurring. Typical of such a current differential transformer in such a ground fault protection scheme is described in the U.S. Pat. No. 3,213,321 issued to C. F. Dalziel.
The utilization of a current transformer for sensing fault conditions has inherent disadvantages such as the associated electronic circuitry required to be coupled to the current transformer for amplifying the sensed signal and the cost of the current transformer.
Some prior art shock prevention devices which attempt to avoid the use of such differential transformer utilize switching coils or relays which when a dangerous leakage occurs in the load, causes the biasing of various transistors which in turn disconnect the line voltage from the operator. Such attempts were neither accomplished in a fail-safe mode nor were they in the environment of portable appliances. Typically, they necessitated permanent type connections at the power source and/or extensive additional circuitry.
Finally, prior art ground fault circuit interrupters operated only when an actual fault (e.g. shock) occurred and not when the potentially dangerous conditions of the operator of the apparatus having the protective circuitry being too closely coupled to either the associated AC conductors or ground. The latter mode of operation for a protective circuit is most desirable since it disconnects prior to the actual shock and teaches the operator of the apparatus not to use apparatus of this type when he or she is contacting ground or is dangerously close to the hot side of the line.
A human being is always capacitively coupled to the environment. Ground being the most prevalent thing around, one is usually coupled closer to ground than to any other potential. The only occurrence which can upset this basic balance is if one is touching a conductor directly. Contacting a conductor in this way brings one's impedance toward the potential of that conductor. This impedance level depends on the contact resistance of the person. Dry, the contact resistance is usually 50,000 ohms to 500,000 ohms, wet; it can be as low as 1,000 ohms. The circuit of this invention requires the operator of the apparatus having the protective circuitry to contact the touch grid of the apparatus while the operator is at the normal impedance (i.e., not in direct contact with ground or other potentials) in order that the apparatus will operate. If the operator lets go of the touch grid or touches ground or a live potential the circuit turns the apparatus off.